Microchip lasers generally comprise a diode pump source and an etalon of gain material forming a resonator cavity. Such lasers are very small but have high efficiency and high power density at the output. A composite microchip laser can have a frequency doubling material such as KTP optically linked by index matching materials to the gain material so as to produce a visible output. Typically a composite microchip laser has end coatings which are highly reflective at the desired fundamental frequency, and highly transmissive at other frequencies; this ensures that the gain at the fundamental frequency is maximised. For example the desired fundamental frequency may have a wavelength of 1064 nm and accordingly end coatings which are highly reflective at that frequency are provided. Laser light at other frequencies, for example those having wavelengths of 1340 nm and 916 nm, are allowed to escape from the resonator cavity by the use of highly transmissive end coatings.
By the use of a suitable non-linear material, the frequency of laser light from the gain material can be doubled, thereby halving the wavelength and producing an output which is visible. For example frequency doubling of laser light at 1064 nm results in an output at 532 nm, which is green and readily visible.
One problem in the manufacture of commercial composite microchip lasers is the reduction in optical losses at the junction between the gain material and frequency doubling material. Reflection losses and light scatter can be rather high at the junction which can be formed of matching fluid such as halo carbon oil.
Another problem in the manufacture of composite microchip lasers is to hold the gain material and frequency doubling material in register. This problem is made more difficult to solve because the component parts of the cavity are very small, the gain material typically having an area of only 3 mm square and a thickness of about 1 mm. Yet a further problem is to accurately hold the microchip laser on a mount or chassis relative to the diode pump source in such a manner that internal stresses are not induced in the resonator cavity due to e.g. thermal effects.
It has been proposed to use an organic material such as an oil between adjacent end faces of the gain material and frequency doubling material. Such a material can eliminate intra cavity voids and thereby reduce reflections. The material also provides good thermal contact between the materials. In the case of oil however, the surface tension effects may be insufficient to maintain the component parts in registration over an extended period of use, the change in registration resulting in lowered output power and increased noise.
It has also been proposed to hold the gain material and frequency doubling material together using an optical cement. Such an arrangement has good thermal conductivity and provides the necessary mechanical rigidity to maintain registration of the materials. However the very high power density of e.g. 10 MW/cm.sup.2 at the fundamental wavelength is sufficient to degrade the cement over a period of time, leading to darkening of the cement and subsequent failure of the laser.
Neither of these assembly methods provides an effective solution to the problem of mounting the resonator cavity on a chassis. A mounting method should be reliable, repeatable and suitable for mass production techniques whilst not thermally isolating the laser or introducing additional parasitic losses which could reduce efficiency of the laser.
A further problem which has been identified in intra-cavity frequency doubled lasers is the existence of amplitude instabilities in the output, especially at high pump power. Amplitude instabilities in these lasers are the result of more than one axial mode oscillating within the resonator cavity. These axial modes may have various polarisations, and can couple together and produce chaotic amplitude fluctuations. In order to produce an intra-cavity frequency doubled laser with reliable, stable output amplitude it is necessary to constrain the laser to oscillate on one axial mode with a well defined polarisation state, and to inhibit oscillation on all other modes.
In the past this has been performed by inserting discrete frequency selection elements and polarisers within the cavity. These components are bulky, and consequently this conventional approach to wavelength discrimination in larger laser cavities cannot be applied to the monolithic format.
The problem of instabilities in intra-cavity doubled lasers is well known in the art and is known as "The Green Problem". These amplitude instabilities inhibit the application of green lasers to fields such as reprographics where green light, being a member of the most commonly employed set of additive primary colours, is used extensively in scanning across a photosensitive plate, amplitude modulation being used to create a desired pattern. Any random or periodic noise on the output of a green laser so used would give rise to "banding" on the image formed on the photosensitive plate, which is clearly undesirable.